In the manufacture of electronic devices such as television receivers, it is desirable to utilize solid state components to the greatest extent possible in order to realize the advantages inherent in such components.
One of the circuits in the color television circuit requiring a relatively large number of components is the chroma demodulator section of the receiver. This portion of the television receiver is used to separate the color difference signals present in the NTSC color television signal. This signal includes a wideband brightness luminance (Y) signal, and a modulated subcarrier signal of approximately 3.58 megahertz. The subcarrier signal is phase and amplitude modulated by color difference signals (R - Y, B - Y and G - Y), so that different phases of the subcarrier each represent the hue of an image portion and the subcarrier amplitude at that phase represents the saturation of the hues. A monochrome receiver visibly reproduces only the white component.
In prior art color television receivers there are several basic problems associated with the use of integrated circuit chroma demodulators. The first problem generic to the use of the integrated circuit chorma demodulators in prior art is that only two demodulators are used. One is used to demodulate the R - Y and the other to demodulate the B - Y color difference signal. A matrix is used to obtain the G - Y signal. Because the matrix is fixed within the prior art integrated circuits, it cannot be varied to meet the needs of different color picture tubes. More specifically, the gain and phase angle associated with the green color signal output is fixed. Thus, as the phosphorus, for visually displaying a color, changes in the color picture tube a different phase angle is required, therefore, the prior art integrated circuit has to be modified to meet the demands of every new picture tube.
Another problem relating to prior art integrated circuit demodulators is that the output direct current (DC) voltage level is not well defined. The voltage level varies with the supply voltage and with temperature. This is undesirable because a small voltage change at the output of the demodulator is amplified and would result in large changes at the cathode of the color picture tube. If this voltage change is a straight DC level shift a brightness variation would be viewed at the picture tube. If it is a differential shift, a change in tint would be evident. For example, the red cathode ray gun might change while the blue and green guns remain constant. This would then result in a redish tint to the picture. A differential change means a relative change between the three outputs.
A further problem with prior art integrated circuit demodulators relates to background adjust of the new unitized gun cathode ray tubes which are now used for color television sets. In the past the color picture tube structure had separate screen grids which allowed the television manufacture to perform the background adjustment or the gray scale tracking adjustment at the screen grids of the color picture tube. However, the new unitized gun structure has the screen grids tied together so that the background adjustment can no longer be made at the color picture tube. Therefore, the background adjustment must be done at some other point, which is made difficult by present integrated circuit demodulators because their DC output levels are fixed and cannot be varied.
A still further problem created by prior art integrated circuit demodulators relates to the difficulty with which their outputs can be DC restored in the video output portion of the color television receiver. Direct current (DC) restoring is that function by which the DC offsets, which might be present in the demodulator and video output, are removed by clamping each of the three outputs, red, blue and green, to the same DC level at the cathode of the color picture tube. This function requires that a well defined pulse be present in each of the three outputs from the integrated circuit demodulators so that the pulse may be clamped to define the DC level in the video output stage. Prior art integrated circuit demodulators do not provide such a pulse, making it very difficult for them to be DC restored in this fashion.
A need exists to develop a solution to the problems associated with prior art integrated circuit demodulators caused by direct current voltage level changes, matrixing of the chromiance signal and to changes in the phosphorus material of the color picture tube. Moreover, some prior art demodulator configurations are not flexible enough to be used with a unitized color picture tube.